1. Field of the Invention
The present invention relates to methods for increasing the productivity of an existing well. More particularly, the invention relates to methods for under-reaming a wellbore. More particularly still, the invention relates to methods for under-reaming a wellbore in an under balanced condition to reduce wellbore damage.
2. Description of the Related Art
Historically, wells have been drilled with a column of fluid in the wellbore designed to overcome any formation pressure encountered as the wellbore is formed. This “overbalanced condition” restricts the influx of formation fluids such as oil, gas or water into the wellbore. Typically, well control is maintained by using a drilling fluid with a predetermined density to keep the hydrostatic pressure of the drilling fluid higher than the formation pressure. As the wellbore is formed, drill cuttings and small particles or “fines” are created by the drilling operation. Formation damage may occur when the hydrostatic pressure forces the drilling fluid, drill cuttings and fines into the reservoir. Further, drilling fluid may flow into the formation at a rate where little or no fluid returns to the surface. This flow of fluid into the formation can cause the “fines” to line the walls of the wellbore. Eventually, the cuttings or other solids form a wellbore “skin” along the interface between the wellbore and the formation. The wellbore skin restricts the flow of the formation fluid and thereby damages the well.
The degree which a wellbore is lined with particulate matter is measured by the “skin factor”. The skin factor is proportional to the steady state pressure difference around the wellbore. A positive skin factor indicates that the flow of hydrocarbons into a wellbore is restricted, while a negative skin factor indicates enhanced production of hydrocarbons, which is usually the result of stimulation. The skin factor is calculated to determine the production efficiency of a wellbore by comparing actual conditions with theoretical or ideal conditions. Typically, the efficiency of the wellbore relates to a productivity index, a number based upon the amount of hydrocarbons exiting the wellbore.
One method of addressing the damage described above is with some form of hydraulic fracturing treatment. For example, in an “acid frac”, hydrochloric acid treatment is used in a carbonate formation to etch open faces of induced fractures. When the treatment is complete, the fracture closes and the etch surfaces provide a high conductivity path from the reservoir to the wellbore. In some situations, small sized particles are mixed with fracturing fluid to hold fractures open after the hydraulic fracturing treatment. This is known in the industry as “prop and frac”. In addition to the naturally occurring sand grains, man made or specially engineered proppants, such as resin coated sand or high strength ceramic material, may also be used to form the fracturing mixture used to “prop and frac”. Proppant materials are carefully sorted for size and sphericity to provide an effective means to prop open the fractures, thereby allowing fluid from the reservoir to enter the wellbore. However, both the “acid frac” and “prop and frac” are very costly procedures and ineffective in lateral wells. In addition, both methods are unsuccessful in removing long segments of wellbore skin. Additionally, both methods create wellbore material such as fines that may further damage the wellbore by restricting the flow of the reservoir fluid into the wellbore. Finally, both methods are difficult to control with respect to limiting the treatment to a selected region of the wellbore.
There is a need, therefore, for a cost effective method to remove wellbore skin to recover and increase the productivity of an existing well. There is a further need for a method to remove long segments of wellbore skin without causing further damage to the wellbore by restricting the flow of the reservoir fluid into the wellbore. There is yet a further need for a method to remove skin within a selected region of the wellbore. There is even yet a further need for an effective method to remove wellbore skin in lateral wells. Finally, there is a need for a method that will not only remove wellbore skin but also create negative skin, thereby enhancing the production of the well.